Detection systems whose operation is based on monitoring the change in frequency of an active circuit element have widespread applications, examples of which are frequency hoppers, phase locked loops, and annalyte sensors using surface acoustic wave devices (SAW devices). Phase locked loops and frequency hoppers must continually or quickly monitor the frequency of a circuit. SAW based annalyte sensors employ a SAW device in a stabilized loop oscillator, and have disposed on the SAW a polymer coating which is adsorptive of a chemical of interest. If such a chemical is present, the polymer absorbs it, thereby increasing the polymer""s mass. The increased mass presses on the SAW, changing its acoustic properties, among which is its resonant frequency. By detecting frequency shift, one can infer the presence of, and concentration of, the chemical, and infer total exposure to the chemical.
For the last eighteen years of SAW chemical sensor development, the focus has been concerned around the development of a sensitive, reversible polymer coatings, rather than developing better detection circuits. With the maturity of the coating technology, SAW sensors found wide acceptance as early warning devices of the presence of harmful chemical agents for military and civilian personnel, and as chemical detection devices in law enforcement and elsewhere. These devices are attractive because of their small size, ruggedness, high sensitivity, and large dynamic range. However, current SAW sensors are based on continuously operated loop oscillators, in which the SAW device is the resonant element, which consume excessive amounts of power because of the DC bias required. Further, these circuits measure change in resonant state of the SAW element by taking a large number of digital samples of the SAW""s output, and counting the number of zero crossings per unit time, or processing the samples in the frequency domain by use of a Fast Fourier Transform, which also consumes a great deal of power. This makes SAW based detection circuits undesirable for applications in which a user must carry the device on his person, because the high power consumption of loop oscillators would not permit operation for any appreciable length of time using ordinary batteries. Moreover, continuously operated loop oscillators can require frequent tuning to maintain circuit oscillation, and excessive loading as the polymer absorbs more chemical can cause the oscillation to cease entirely.
Accordingly, an object of the invention is to prevent mass loading or stiffening of the SAW detectors and the like from causing termination of circuit operation.
Another object is to reduce power consumption in systems using resonant elements sufficiently to permit such a circuit to be carried on one""s person comfortably.
Another object is to do the foregoing in the time domain.
Another object is to reduce power consumption sufficiently to permit long term operation of such a circuit (i.e. days or weeks) using only simple and small batteries.
In accordance with these and other objects made apparent hereinafter, the invention concerns a system having a resonator, a pulse generator, and a detector, in which the detector receives the response of the resonator to at least one pulse from the pulse generator, produces one or more digital samples of the response, and from this determines, a parameter of the resonator determinative resonant frequency.
Because this does not rely on maintenance of circuit oscillation, mass loading of the resonator cannot interfere with system operation, and the system can detect much greater amounts of sorbed chemical.
Because interrogation is done by pulsed ringing of the resonator, rather than by maintenance of a continuous oscillation through the resonator, the amount of power uses is greatly reduced, such that one can reduce the size of a package containing the invention sufficiently to permit the package to be carried on one""s person comfortably, and to operate sufficiently long (i.e. days or weeks) to permit using only simple and small batteries as a power source.
Because detection is done in the time domain, rather than in the frequency domain, one does not have to use Fast Fourier transforms or frequency counting schemes, which require a large number of samples to operate properly, and hence consume a great deal of power in so doing.
These and other objects are further understood from the following detailed description of particular embodiments of the invention. It is understood, however, that the invention is capable of extended application beyond the precise details of these embodiments. Changes and modifications can be made to the embodiments that do not affect the spirit of the invention, nor exceed its scope, as expressed in the appended claims. The embodiments are described with particular reference to the accompanying drawings, wherein: